The 'Nephalem were the first humans upon Sanctuary, created as a result of the union between Angels and Demons. They turned out to be immensely powerful, and for a while the High Heavens, with Tyrael in the front wanted to destroy them all.

When the Angiris Council could see that humans could do great good, they left them, and since then Tyrael has been helping humans fighting demons. They also created the Worldstone, which suppressed the nephalem's power, and slowly eradicated all traces of their omnipotence, save for a few.

Early humans were wiped of their memories as omnipotent beings, and started creating societies as humans do, spreading across the world, creating different cultures.